Anionic surfactants, particularly the sulphonates such as linear alkylbenzene sulphonate (LAS) and the sulphates such as primary alkyl sulphate (PAS), are the key ingredients of modern laundry detergents, providing excellent detergency performance on a wide range of soils and stains.
For optimum performance, especially in the handwash or in top-loading washing machines where foaming is not a problem, it is desirable to provide a high level of such anionic surfactants in the powder. However, the amount that can be incorporated may be limited by process and powder property requirements.
Most laundry detergent powders, whether of high or low bulk density, contain a “base powder” consisting of composite granules of surfactant and inorganic builder (and other) salts, prepared either by spray-drying a slurry in a spray-drying tower, or by mixing and granulation (non-tower processing) in a high-shear mixer/granulator. Other lesser ingredients that are sufficiently robust to survive these processes may also be contained in the base granules, while more sensitive ingredients such as bleaches and enzymes are subsequently admixed (“postdosed”).
Surfactants are mobile organic materials and the amount that can be incorporated in a base powder, whether spray-dried or non-tower granulated, without causing processing difficulties, and products with poor flow and stickiness, is limited. It is therefore known to top up the anionic surfactant content of laundry powders by postdosing high-active anionic surfactant granules.
High active surfactant granules are disclosed, for example, in WO 96 06916A and WO 96 06917A (Unilever). These granules are prepared by flash drying. LAS granules prepared by this route typically contain up to 80 wt % LAS, together with a salt such as zeolite or sodium tripolyphosphate. Granules without salt may be prepared, but are highly hygroscopic. PAS granules may contain up to 100 wt % PAS without developing hygroscopicity problems, but PAS does not have the robust detergency profile of LAS across a wide range of wash conditions.
The present inventors have now found that a composite particle of LAS and PAS containing little or no inorganic salt can be prepared which combines low hygroscopicity with an excellent detergency profile. This particle is very useful in the formulation of a range of high-active solid detergent products.